welcome to the forums (IMG:
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this is a bassar situation, there is obviously something wrong with your clutch or (more likely) the gearbox. the problem only seems to happen when the vehicle gets hot and has been driving for a few miles, get it back to your dealer and tell them this and demand that it is taken for a long test drive, it would be better if you went with them. the most obvious thing that comes to mind........is there any oil in the gearbox?
your car is still under warranty. if you don't get any satisfaction from your dealer phone hyundai U.K. direct and complain